With apologies to her sisters Mary-Kate and Ashley, Elizabeth may be the biggest Olsen of all. For proof positive, look no farther than her star turn in this month’s Martha Marcy May Marlene.

On her third day at the Sundance Film Festival this past January, Elizabeth Olsen’s life changed. She was the star of two movies in Sundance—Silent House, which is a horror film, and, more importantly, ­Martha Marcy May Marlene, the riveting, chilling, psychologically complex story of a damaged young girl who escapes from a cult. Martha had resonance—intended or not, the film had parallels to current religious and cultlike political movements such as the Tea ­Party, as well as echoes of Charles Manson—but at first the talk of the festival was focused on Olsen’s performance. Olsen, who is 22, had only started auditioning for films in February 2010, and was vaguely known, if she was known at all, as the youngest Olsen, Mary-Kate and Ashley’s little sister. The twins had named a clothing line, Elizabeth and James, after her and their brother, but until Sundance, until Martha Marcy May Marlene, few knew what Lizzie, as she is called, looked like—or that she could act.

After the initial showing of Martha at the Eccles Theatre, word spread immediately around the festival that a star might just have been born. “For the first two days at Sundance, nobody noticed me,” Olsen told me nearly six months later. “The third day was completely different. Suddenly, walking down the street was an experience. I could hear people whispering about me or staring a little too long. It was so weird that it made me laugh. In every picture from Sundance, I am laughing.”

Olsen was laughing now—a kind of joyous, carefree laugh that is consistent with her personality. Unlike her character in Martha, she is decidedly not fragile, not conflicted, not haunted, not troubled. Instead, she is poised, engaged, and un-neurotic, which is unusual for someone so young. In fact, despite her round, clear eyes and open face, it’s easy to forget that Olsen is still in college at New York University; she has the sophistication and perspective of someone much older who managed to stay unjaded.

We were eating lunch in one of her favorite health-food restaurants, around the corner from her apartment in downtown Manhattan. ­Olsen was wearing black shorts that showed off her long, bare legs, a sheer orange top, and flat Céline loafers. Her brown hair was pulled back in a ponytail, and the streaming sunlight seemed to amplify the glow of her face. “Lizzie takes the light in an amazing way,” Sean Durkin, the director of Martha, said. “It’s not typical. She can look like a stunning, old-Hollywood actress one minute, and then, she can turn away from the light and blend in like a regular girl.”

FROM WHEN SHE WAS EIGHT, Olsen had wanted to act. She grew up in Los Angeles, and studied with Mary-Kate and Ashley’s on-set coach from Full House. Olsen started auditioning in the fourth grade, landed a commercial, and then at 10 decided to quit. “My dad wanted all the girls in our family to be independent,” Olsen explained, as she drank a dark green shake of kale, spinach, and other assorted leafy vegetables. “All the women in the family have this crazy drive. As a kid, I always liked auditioning, but after five auditions or so I missed ballet and after-school sports. My father told me to write out a pros and cons list: acting vs. ballet and sports. I needed to have reasons to decide to do one or the other.”

At 16, Olsen, who excelled academically at Campbell Hall, a private school in L.A., began studying Russian theater. While immersing herself in the Soviet theatrical tradition, Olsen had an epiphany: She definitely wanted to pursue acting. She started performing in school plays and decided to apply to NYU’s theater department. During her sophomore year in 2009, she auditioned to be the under­study for the Broadway play Impressionism, starring Joan Allen and Jeremy Irons. “For an entire year, I was the understudy to Margarita Levieva,” Olsen said. “And I met my agent, Rhonda Price, then. I didn’t really want to meet her—I wanted to finish school. It all happened so fast and naturally.”

It was decided between Olsen and Price—almost ordained—that Lizzie would concentrate on theater and movies rather than television, which is where her sisters had become famous. Their lives in the public eye have not made Olsen wary exactly—just informed and cautious. “Theater seemed safe,” Olsen said. “In the theater, you’re sort of hidden. Most paparazzi are not after stage actors. At first, movies were really frightening to me. In my mind, if you’re in a movie, you’re officially out there.” She paused. This was an uncharacteristically dark thought. “But then you realize, who cares?” Olsen continued, suddenly brightening. “I just want to do what I want to do.”

She started reading scripts—eight or so a week. “I auditioned for everything. I love waiting rooms before auditions. Everyone is so nervous and looking each other up and down. Sometimes, I just look at everyone else freaking out, speaking to themselves. I had a teacher who said every time you audition, it’s another opportunity for someone to see you act. The casting agents and directors don’t want you to suck, because if you suck it makes their day longer. And if you’re not what they want, that’s not your fault.”

Luckily, an abundance of talent has smoothed Olsen’s path and allowed her to remain unburdened by thoughts of rejection: She landed her first movie, Peace, Love & Misunderstanding, only six months after she began auditioning. She was about to start filming in upstate New York when she read Martha Marcy May Marlene. “I was obsessed,” Olsen recalled. “I thought, This is the kind of part I want to play when I’m older. With Martha, I could do all the things I want to do later, but now.”

Martha was written by Durkin, who is one-third of Borderline Films, a Brooklyn-based collective run with Antonio Campos and Josh Mond. Their independent film company has an interesting, utilitarian structure: Durkin, Campos, and Mond, who bonded and formed their company as film students at NYU, each writes, produces, and directs. The three men work on each others’ movies in every capacity—from finding financing to honing the script to casting to overseeing the production to directing. For example, while Durkin wrote and directed Martha, Mond produced and Campos consulted throughout the movie. “They all split duties,” Olsen recalled. “Antonio would give his two cents about how to stage certain scenes in the movie. All three work really closely together.”

Durkin, who is 29 and has a calm, thoughtful manner, became interested in making a film about a cult in 2007. “A friend of Josh’s had been in a cult,” Durkin explained to me when I went to meet the Borderline trio at Sweetwater, a restaurant in Williamsburg near their new office, which the $1.6 million Sundance sale of Martha to Fox Searchlight had made possible. “I listened to her story and I started to do research. I came across a passage in one book about a girl who ran away from a cult after the group turned violent. They gave her money, wished her well, and then she disappeared. I tried to imagine what those first few weeks after leaving the cult were like. That was the start of Martha.”

The film, which deftly jumps back and forth in time, follows Martha (her many names reflect her fractured identity as she tries to readjust to life outside the cult). Her sister (played by Sarah Paulson) and brother-in-law (Hugh Dancy) take her into their large, waterfront home, but her delicate emotional state and the real and imagined threat from the rejected cult members unravel Martha. The movie convincingly shows the lasting, destructive power of a charismatic, manipulative leader (brilliantly played by John Hawkes) over an insecure girl in search of herself.

“I wasn’t trying to make a political statement,” Durkin told me, after I suggested that the film had, perhaps, a larger symbolism. “But, in looking back, I think Martha may have come out of my own religious history. I went to a strict Anglican school in England and I felt isolated. We had to go to church every morning, and I found myself chanting without knowing what the meanings of the prayers were. I wanted to tap into that emotion—the feeling of belonging and still feeling lost.”

Durkin saw around 75 actresses for the part of Martha. “You know it when you see it,” he said. “I texted Antonio after Lizzie’s first reading: ‘I think this girl might be something special.’” Olsen made an impression immediately: Even before speaking, she had arrived at the audition with huge suitcases. “I was moving out of my apartment, on my way upstate for Peace, Love & Misunderstanding, and I was carrying these three bags like a crazy person,” she told me. “Sean always says that’s why I got the part—I seemed crazy.”

Even during the filming of tense and depressing scenes, Olsen remained almost giddy. “I had to ask her to stop singing during takes,” Durkin said. “She’d be doing a really upsetting scene, and she and Sarah Paulson would be doing show tunes. I had to ask her to stop.” Not only was Olsen turning cartwheels (literally) between takes, but she was also filming another movie. “I drove myself between Peace, Love & Misunderstanding and Martha,” Olsen said, still sounding gleeful. “But I keep myself very separate when I’m acting. I know what’s me and what’s the character. Martha never got to me—it was intense, but it’s important to keep yourself separate when you’re young and impressionable.”

In May, four months after Sundance, Olsen stood on the Croisette in Cannes, about to attend the premiere of Martha Marcy May Marlene. It was a Sunday around 9 p.m., and it was still light out. Olsen had arrived two days prior from New York; checked into her hotel, the Trois Quatorze; and, ever practical, immediately realized that she would have to do her makeup in her agent’s room. “My room had a Parisian theme,” Olsen told me at a small dinner Fox Searchlight threw for Martha. “Everything was red—the lightbulbs were pink. Rhonda’s room had an Asian theme. Everything was white in Asia. I put my makeup on there.”

During Cannes, Olsen did dozens of interviews and posed for countless pictures. The international press, which congregates in Cannes, is often contrarian and peculiar, regularly asking questions that are completely unrelated to the films. Of course, they asked Olsen about her sisters (she demurred politely) and, more provocatively, if Martha was meant to be about the power and impossibility of utopian societies.

At Cannes (and Sundance), there were other films that dealt with cult-like religious fanaticism. Most notably, Take Shelter (which went on to win the Grand Prize at Cannes Critics’ Week), a psychological study in which a man systematically destroys his life when he starts to believe that the end of days is imminent. He seeks a god-like stature, but is able to persuade only his wife and small child (a cult of two) to move into a storm shelter for safety. Like Martha, Take Shelter offers the idea of salvation through isolation and devout belief; that same clannish, xenophobic, paranoid-but-superior mentality is also the trademark of today’s American political sub-tribes. Like cult members, they crave a messianic leader who preaches dogma and re-affirms the “specialness” of the group. The journalists tried to press Durkin and Olsen about these sociopolitical overtones, but the filmmakers were resistant. Which was okay with the press corps—their more intellectual queries were trumped by their passion for Olsen’s performance. They veered away from the serious to swoon: Cannes loves to anoint a star.

Since Sundance is strictly puffers and mufflers, this was Olsen’s first black-tie premiere. She wore a sleek black gown from The Row and looked rather like a Twenties-era ingenue. “This doesn’t feel like reality to me,” she said, before heading to the red-carpeted steps that led to the theater. “And that makes me laugh. I thought I would find the photographers and the press irritating, but it struck me as funny. We made this little movie, and now we’re standing in pretty outfits and smiling at the camera. You have to laugh.”

With that, she smiled. One can only hope that, as time goes on and the world does not seem quite so easy, Olsen won’t lose her sense of joy and optimism. As the paparazzi called out Lizzie’s name—a name they didn’t know a few days earlier—and asked her to look left and then right, Olsen briefly looked backward, at the street where a small group of fans had gathered. She waved, they took her picture, and she turned back to the theater and headed upstairs. It was the only way to go.